A spherical plug valve, also known as a ball valve, includes a body structure within which is movably located a plug member or ball having a through passage and defining a spherical sealing surface. The ball may have integral trunnions or may have trunnions and actuator stems operatively connected thereto in any suitable manner. Ball valves are provided with seat assemblies which are typically movable within the valve body and which establish sealing engagement with the spherical sealing surface of the ball. The sealing surfaces of the seat members are of circular form and are typically of tapered or arcuate cross-sectional configuration closely or precisely corresponding to the curvature of the spherical sealing surface of the ball. In essence, the sealing surfaces of the seat members capture the spherical sealing surface of the ball, consequentially the ball may be difficult to remove from the valve body in the event servicing becomes necessary.
In most cases ball valves are designed with two or three piece body structures, enabling body sections to be separated for ease of removing the valve element. However, two or three piece body structures are not entirely satisfactory because body leakage can occur at body seals and the valve body structure may not be capable of withstanding significant line stresses which are prevalent in piping systems controlled by such valves. Also, two or three piece body style valves require removal from the piping system for maintenance, thus increasing cost of time for maintenance and longer system shutdown time.
Top entry trunnion type ball valves have been in use for a considerable period of time and are considered to minimize many of the undesirable features of ball valves having sectioned body structures. The advantages of top entry ball valves include a one piece body structure to withstand line stresses with no body joints for possibility of leakage. The seat rings of ball valves establish sealing contact with the spherical surface of the ball. This requires that the end faces of the seat rings extend over the spherical surface of the ball in both closed and open positions of the ball. The ball is therefore contained within the inner circumferential sealing point of the seat rings. It is difficult therefore to achieve retraction of the seat assemblies sufficiently to permit installation or extraction of the ball. This disadvantage has significantly retarded wide use of top entry trunnion ball valves in the past. Top entry style trunnion ball valves do not require removal from the piping system for maintenance, thus minimizing cost of time for maintenance and system shutdown time.
Top entry trunnion ball valves are normally designed with trunnions or journals integral with a spherical shaped ball at the upper and lower portions of the ball. The ball includes a through port opening capable of axial registry with the flow passage through the valve body. Both trunnions or journals are rotatable within trunnion or journal bearings centrally located in the lower area of the body structure and in the bonnet or cover forming a closure for the access opening of the body. Two seat rings are oppositely located in seat recesses in the body at each side of the ball, being axially aligned with ball and body flow passages. The seat rings are designed to have sealing contact with the spherical sealing surface of the ball. This requires that the end faces of the seat rings extend over the spherical surface of the ball in both open and closed positions of the ball, therefore the ball is contained within the internal circumferential sealing point of the seat rings and is thus captured by the seat rings. The seat rings are preloaded against the sealing surface of the valve ball by the force of various types of springs positioned behind the seat rings. This spring preload is required to create initial sealing contact of the seat rings against the ball for further line pressure responsive seat sealing.
Because of the spring preload requirement, means must be provided for retraction of the seat rings against the spring forces to provide sufficient clearance for assembly and disassembly of the ball without damaging the spherical sealing surface of the ball by any obstructing conditions.
Top entry trunnion ball valves presently being manufactured and marketed require expensive specially designed tools to retract the seats for assembly or disassembly of the ball, or require a minimum of four holes through the valve body wall for insertion of tools to achieve retraction of the seats. These special type tools are typically of the type that maintenance personnel will not ordinarily possess. Additional holes through the valve body wall require plugging and present potential hazardous leakage points to the atmosphere which could develop due to improper plugging or due to corrosion of the plugs or wall structure adjacent the plugs. Also, manufacturing costs are significantly increased when such valve body plugs are employed.